Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier.
The process of preparing inks from pigments commonly involves two steps: (a) a dispersing or milling step to break up the pigment to the primary particle, and (b) a dilution step in which the dispersed pigment concentrate is diluted with a carrier and other addenda to a working strength ink. In the milling step, the pigment is usually suspended in a carrier (typically the same carrier as that in the finished ink) along with rigid, inert milling media. Mechanical energy is supplied to this pigment dispersion, and the collisions between the milling media and the pigment cause the pigment to deaggregate into its primary particles. A dispersant or stabilizer, or both, is commonly added to the pigment dispersion to facilitate the deaggregation of the raw pigment, to maintain colloidal particle stability, and to retard particle reagglomeration and settling.
Although there are known in the prior art a wide variety of dispersing agents for pigmented ink jet inks, they are not without certain problems. For example, many dispersing agents are very selective as far as being able to disperse pigments to sub-micron size. In many instances each class of pigments may require a specific dispersing agent. Another problem encountered with polymeric dispersing agents is that they tend to impart an undesirable high viscosity to the resulting inks. Thus there is a continuing need for improved dispersing agents for pigmented inks.
The ink jet inks are employed in imaging processes which involve the application of liquid ink droplets in a pixel-by-pixel manner to an ink-receiving element. There are numerous schemes which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to the ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed on to the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation.
In any of the known ink jet printing methods, the properties of the inks are critical to the overall performance of the system. One such ink property which has been found to have a profound impact on both the drop ejection process and on the interaction of the ejected ink droplet with the receiving surface and with other inks is surface tension. The control of surface tensions in aqueous inks is most commonly accomplished by additions of small amounts of additives known as surfactants. Sodium N-oleyl-N-methyl tauride is described as an anionic surfactant in pigment dispersions containing phthalocyanine pigments. The dispersion are suggested for use in making, among other things, a nonaqueous printing ink.